1. Field of the Invention
The present invention relates to a print paper detecting circuit installed on a printer, and more particularly to a print paper detecting circuit which detects whether a sheet of print paper is supplied at the printing position using an optical sensor.
2. Description of the Related art
A printer prints on various types of print paper supplied from various paper feed paths, such as a cut sheet of print paper which is manually supplied via a paper insertion path or is automatically supplied from a cut sheet feeder (CSF), or a continuous print paper which is supplied from a pin tractor mechanism. The printer detects whether the paper is supplied at the printing location and controls a paper alarm LED to emit a alarm signal and stops the printing operation when the paper is not supplied at the printing position. It is well-known that an optical sensor, such as a photoreflector, may be used to detect the paper.
FIG. 14 shows a sectional side elevation of the general structure of the printer.
In FIG. 14, the cut sheet of print paper 41 is inserted on a guide 42 by hand or is supplied from a cut sheet feeder (CSF) (not shown), and is fed between a platen 43 and a feed roller 44. A photoreflector 45 is mounted on the side of the guide 42 opposite that which the platen 43 is mounted and adjacent an opening 42a in the guide 42 so that light emitted by the photoreflector 45 will strike either the platen 43 or a sheet of paper 41, if present adjacent the opening 42a. The cut sheet of print paper 41 is inserted between the platen 43 and the feed roller 44 and is fed along a guide 46 between the platen 43 and a printing head 47 due to rotation of the platen 43. After printing on the cut sheet of paper 41 by the printing head 47, the paper 41 is inserted between the platen 43 and a bail roller 48 and is fed to a output opening 49.
A paper alarm LED 50 is installed o the front of the printer.
A paper separator 51 is provided to permit an already printed sheet 41 being fed out of the printer from accidently again entering the paper feed path.
The paper separator 51 and a rear cover 52 are removable so that a cut sheet feeder (CSF) can be installed on the printer if desired.
FIG. 15 shows the print paper detecting circuit having a photoreflector 45 as shown in FIG. 14.
In FIG. 15, a light emitting diode PHD of the photoreflector 45 is serially connected to a current limiting resistance R1 which determines the current intensity of the light emitting diode PHD. Supply or source voltage +E is supplied to one end of the series connection of the resistance R1 and the light emitting diode PHD while the other end is connected to ground, i.e., the other terminal of the voltage source. The emitter of a phototransistor PHTR of the photoreflector 45 is grounded and the collector of the phototransistor PHTR is connected to one end of a load resistance R2 whose other end is connected to the source supply voltage +E. The output voltage Vo at the collector of transistor PHTR is changed to a logical level by an input buffer BUF which provides an output to a common controller (not shown) as a print paper detecting signal.
In the print paper detecting circuit, if the output current of the phototransistor PHTR is Io and the input current of the input buffer BUF can be ignored, the output voltage (Vo) of the phototransistor PHTR is given by following equation (1): EQU Vo=E-R2.times.Io (1)
When the print paper is not supplied, most of light emitted from the light emitting diode PHD is absorbed by the platen 43 and only a small amount of light reaches the phototransistor PHTR. At this time, if the output current is Iob and output voltage (Vo) is Voh, the output voltage (Voh) is defined by the following equation (2): EQU Voh=E-R2.times.Iob (2)
When the sheet of print paper is supplied, most of light emitted from the light emitting diode PHD is reflected from the surface of the paper and arrives at the phototransistor PHTR. At this time, if the output current is Iow and output voltage is Vol, the output voltage (Vol) is given by the following equation (3): EQU Vol=E-R2.times.Iow (3)
The relationship between the output current (Iob) and output current (Iow) is Iob &lt;Iow on the basis of the difference in reflectance between the print paper sheet 41 and the platen 43. Moreover, supply voltage (+E) and the load resistance is preset so that the relationship between the threshold voltage Vth of the input buffer BUF and the output voltage is Voh&gt;Vth&gt;Vol. Therefore, the output voltage from the optical sensor is changed to a logical level by the input buffer BUF and a output signal from the input buffer BUF is either a "0" level when the paper is supplied, or a "1" level when the paper is not supplied. This output signal is sent to the common controller for the printer as the print paper detecting signal. The common controller monitors the print paper detecting signal, detects whether the print paper is supplied at a predetermined position and controls the paper alarm LED, the printing operation and the paper end monitor according to the detected result.
FIG. 16 is a sectional side elevation showing the general structure of the printer of FIG. 14 but with a cut sheet feeder (CSF). The cut sheet feeder (CSF) comprises a hopper 53 for holding the paper sheet, a pick-up roller 55 for feeding a sheet of paper which is in the hopper 53, a frame 54, a spring 56 for pushing the hopper 53 and the paper thereon toward the pick-up roller 55, a stacker 58 for stacking printed paper sheets, and an output roller 59 for feeding the printed paper sheets to the stacker 58.
The pick-up roller 55 has a gear which is selectively engaged with the gear of the shaft of the platen 43 by an electromagnetic clutch, now shown. Therefore, when the gear of the pick-up roller 55 is engaged via the clutch, the pickup roller 55 rotates in synchronization with the platen 43, and when the gear is not engaged, the pick-up roller 55 does not rotate eve when the platen 43 rotates.
The spring 56 pushes the paper sheets into engagement with the pick-up roller 55. One paper sheet at a time is fed from the hopper 53 to the platen 43 by the rotation of the pick-up roller 55, since the spring 56 provides a constant pressure between the paper sheets and the pick-up roller 55.
The output roller 59 has a gear which engages the gear of the platen shaft, rotates together with the rotation of the platen, and then feeds the printed paper sheet to the stacker 58.
The print paper detecting circuit described above has the problem that when external light, such as natural light or illuminating light, enters the inside of the printer and arrives at the phototransistor PHTR and when the energy of the external light is equal to or greater than the energy of the light which is emitted from the light emitting diode PHD and which is reflected from the print paper 41, the print paper detecting circuit effects erroneous detection and the common controller effects erroneous operation.